This invention relates to a glass-ceramic substrate for a magnetic information recording medium used for recording various types of information and, more particularly, to a magnetic information recording disk suitable for perpendicular recording having a high heat resisting property which will enable film forming at a high temperature and also annealing at a high temperature, super flatness of the surface of the substrate, and a low degree of elution of alkali ingredients from the substrate during film forming and annealing processes conducted at a high temperature. In this specification, the term xe2x80x9cmagnetic information recording mediumxe2x80x9d means a magnetic information recording medium in the form of a disk and includes fixed type hard disks, removable type hard disks and card type hard disks used respectively for so-called hard disks for personal computers and other magnetic information recording medium in the form of a disk which can be used for storage of data in, e.g., digital video cameras and digital cameras.
In recently developed magnetic information recording devices which handle data of a large amount such as data of a moving image or voice which is necessitated by developments of personal computers for multi-media purposes and digital video cameras and digital cameras, recording density of a magnetic information recording medium has reached an extremely high recording density exceeding 1 Gb/in2.
Conventional magnetic information recording devices mostly employ the longitudinal magnetic recording system according to which magnetization (i.e., recording) is made in the longitudinal direction. In these conventional devices, however, wavelength of recording, track width and film thickness of a recording medium are reduced as the recording density increases and this causes decrease in magnetic energy of a magnetic material constituting 1 bit resulting in decrease in the signal output level and increase in the signal-to-noise ratio. When the recording density exceeds 10 Gb/in2, there arises a further problem that such a small magnetization gives rise to thermal instability and thereby causes difficulty in the magnetic recording.
In contrast, according to the perpendicular magnetic recording system in which the easy axis of magnetization is perpendicular, the bit size can be significantly reduced and, by capability of having a desired film thickness of a magnetic recording medium (five-fold to ten-fold of the film thickness used in the case of the longitudinal recording system), reduction of the diamagnetic field and an advantageous result caused by the shape magnetic anisotropy can be expected. Thus, according to the perpendicular recording system, the decrease in the recording energy and the problem of thermal instability which are caused in increasing the recording density in the conventional longitudinal recording system can be eliminated and a significant improvement in the recording density over the longitudinal recording system can be realized. For these reasons, recording density of 20 Gb/in2 or over can be easily achieved and study for realizing recording density of 100 Gb/in2 has already been started.
Since magnetization is made in the perpendicular direction with respect to the surface of a recording medium in the perpendicular recording system, a recording medium having the easy axis of magnetization extending in the perpendicular direction is employed instead of a recording medium used in the longitudinal recording system which has the easy axis extending in the longitudinal direction. The most hopeful candidate for a perpendicular magnetic film is a barium ferrite film and other materials which are considered suitable for the perpendicular magnetic film are alloy films including films of Co-xcex3Fe2O3 alloy, Co alloys, Fe alloys and Ni alloys.
A substrate suitable for the recording medium used for the new technique which is different from the conventional longitudinal recording system is required to have the following properties:
(1) An oxide medium such as barium ferrite requires film forming at a high temperature for minimization and growth in the perpendicular direction of crystal grains of the magnetic substance. Further, a recent study has revealed that there is a case where a recording medium should be annealed at a high temperature of 500xc2x0 C. to 900xc2x0 C. Therefore, the substrate material must stand such a high temperature without causing deformation of the substrate or change in the surface roughness.
(2) As the recording density increases in the perpendicular recording system, there is an increasing tendency toward lowering of the glide height of a magnetic head to 0.025 xcexcm or below and there is also a tendency toward employing the near contact recording system or the contact recording system. On the other hand, for utilizing the surface of a recording medium effectively as a data zone, the ramp loading system which does not provide the landing zone as opposed to the currently employed recording system which provides both the landing zone and the data zone attracts the attention of the industry. For coping with these tendencies, the data zone on the disk surface or the entire surface of the substrate must be formed in a super flat surface for enabling such low glide height or even contact recording of the magnetic head.
(3) A substrate for a perpendicular magnetic information recording medium must be free of crystal anisotropy, foreign matters and impurities which adversely affect crystals of the material formed into a film. The substrate must also have a dense, uniform and fine texture.
(4) As the recording density increases in a perpendicular magnetic information recording medium, a magnetic film of a higher precision and a finer texture is required. If Na2O, K2O or an ingredient having an OH group is included in the material of the recording medium, ion of such ingredient diffuses during the film forming process resulting in increased coarseness of grains of the film and deterioration in orientation. The substrate therefore should not substantially contain such ingredients. Further, the substrate should not substantially contain PbO which is undesirable from the viewpoint of environment protection.
(5) The substrate should have sufficient chemical durability for standing rinsing and etching with various chemical reagents.
(6) As the recording density increases in a perpendicular magnetic information recording medium, the bit density and track density increase. As the size of the bit cell is reduced, a coefficient of thermal expansion of the substrate has a great influence. Therefore, a coefficient of thermal expansion within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C. must be within a range from xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to 80xc3x97+10xe2x88x927/xc2x0 C.
As a material for a magnetic disk substrate, aluminum alloy has been conventionally used. The aluminum alloy, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process. As a result, the aluminum alloy substrate is not sufficient in flatness and smoothness as a substrate for the perpendicular magnetic information recording medium. Further, since the aluminum alloy is a soft material and tends to be easily deformed, it is difficult to form into a thin disk. Furthermore, a head crash is likely to occur during a high speed rotation of the disk due to flexion of the disk which will damage the recording medium. Therefore, the aluminum alloy is not a material which can sufficiently cope with the tendency toward the high recording density. Moreover, since the heat resisting temperature of the aluminum alloy during the film forming process which is the most important factor as the substrate for the perpendicular recording system is only 300xc2x0 C. or below, film forming at a temperature above 500xc2x0 C. or annealing at a temperature within a range from 500xc2x0 C. to 900xc2x0 C. will cause thermal deformation in the substrate so that application of the aluminum alloy to a substrate for a perpendicular magnetic information recording medium requiring a high temperature processing is difficult.
As materials for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as soda-lime glass (SiO2xe2x80x94CaOxe2x80x94Na2O) and alumino-silicate glass (SiO2xe2x80x94Al2O3xe2x80x94Na2O). These materials, however, have the following disadvantages:
(1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner and, moreover, the substrate has a low heat resisting property. That is, flatness measured by a specific method of a recording medium made by forming a film of a perpendicular magnetic recording material on a sample of a chemically tempered glass substrate at a high temperature of 500xc2x0 C. or over exhibits a large value exceeding 5 xcexcm. Therefore, there will occur problems of deformation after the film forming process and change in the quality of the chemically tempered layer and untempered layer,
(2) Since the glass contains Na2O or K2O ingredient as an essential ingredient, the glass has the problem that elution of Na ion or K ion from the substrate into the recording material takes place during the film forming process and a barrier coating over the entire surface of the glass becomes necessary for prevention of Na ion or K ion. This prevents stable production of the product at a competitive cost.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, Japanese Patent Application Laid-open Publication No. Hei 6-329440 discloses a glass-ceramic of a SiO2xe2x80x94Li2Oxe2x80x94MgOxe2x80x94P2O5 system containing lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) as main crystal phases. This glass-ceramic is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of xcex1-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 xc3x85 to 50 xc3x85. This glass-ceramic, however, cannot sufficiently cope with the requirement for the low glide height necessitated by the rapidly increasing recording density which requires the surface roughness of 1 xc3x85-5 xc3x85. Further, this glass-ceramic has not sufficient heat resisting property. That is, flatness of the glass-ceramic substrate measured by a specific method after subjecting it to a high temperature environment (500xc2x0 C. or over for 5 minutes or over) is larger than 5xcexcm. There arise, therefore, problems of deformation of the substrate after film forming or annealing and change in the surface roughness.
Japanese Patent Application Laid-open Publication No. Hei 7-169048 discloses a photo-sensitive glass-ceramic made of a SiO2xe2x80x94Li2O system added with Au and Ag and Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk made of a SiO2xe2x80x94Al2O3xe2x80x94Li2O system and containing lithium disilicate (Li2O.2SiO2) and xcex2-spodumene (Li2O.Al2O3.4SiO2). These glass-ceramics, however, have, like the above described glass-ceramic, a low heat resisting property. That is, flatness of the glass-ceramic substrates measured by a specific method after subjecting them to a high temperature environment (500xc2x0 C. or over for 5 minutes or over) is larger than 5 xcexcm. There arise, therefore, the problems of deformation of the substrates after film forming or annealing and change in the surface roughness.
U.S. Pat. No. 5,336,643 discloses low expansion transparent glass-ceramics of a SiO2xe2x80x94Al2O3xe2x80x94Li2O system and U.S. Pat. No. 5,028,567 discloses glass-ceramics of a SiO2xe2x80x94Al2O3xe2x80x94ZnO system. None of these publications considers or suggests the heat resisting property required for a substrate for a perpendicular magnetic information recording medium, namely flatness measured by a specific method after subjecting the substrate to a predetermined temperature environment (500xc2x0 C. or over at 5 minutes or over). Particularly, none of these publications discusses the importance of maintaining a super flat substrate surface after film forming or annealing at a high temperature.
It is, therefore, an object of the invention to provide a glass-ceramic substrate for a magnetic information recording medium which can overcome the above described disadvantages of the prior art substrates and has a high heat resisting property and a super flat surface capable of coping with a high recording density according to the perpendicular recording system in future.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that, in glass-ceramics containing a specific crystal phase or phases as a main crystal phase or phases, a glass-ceramic substrate for a magnetic information recording medium can be found which substrate has flatness of 5 xcexcm or below as measured by a specific method and a coefficient of thermal expansion within a range from xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +80xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C., has fine crystal grains having a diameter within a range from 0.001 xcexcm to 0.10 xcexcm, has a super flat surface after polishing, and has no likelihood of diffusion of Na ion or K ion and that this glass-ceramic substrate is by far superior to the prior art substrates for achieving the object of the invention.
For achieving the above described object of the invention, there is provided a glass-ceramic substrate for a magnetic information recording medium having flatness of 5 xcexcm or below in a state after the substrate is heated to 500xc2x0 C., held for 5 minutes and then cooled.
In one aspect of the invention, the glass-ceramic substrate has a coefficient of thermal expansion within a range from xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +80xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C.
In another aspect of the invention, the glass-ceramic substrate has a surface roughness Ra (arithmetic mean roughness) within a range from 1 xc3x85 to 5 xc3x85 after polishing and Rmax (maximum roughness) of 100 xc3x85 or below after polishing.
In another aspect of the invention, there is also provided a glass-ceramic substrate wherein crystal grains of a crystal phase grown therein have a diameter within a range from 0.001 xcexcm to 0.10 xcexcm.
In another aspect of the invention, the glass-ceramic substrate contains, as a main crystal phase or phases, one or more crystal phases selected from the group consisting of xcex2-quartz (xcex2-SiO2), xcex2-quartz solid solution (xcex2-SiO2 solid solution), xcex2-spodumene (xcex2-Li2O.Al2O3.4SiO2), xcex2-spodumene solid solution (xcex2-Li2O.Al2O3. 4SiO2 solid solution), xcex2-eucryptite (xcex2-Li2O.Al2O3.2SiO2, a part of Li2O being a capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3.2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO).
In another aspect of the invention, the glass ceramic substrate contains, as a main crystal phase, gahnite (ZnAl3O4) and/or gahnite solid solution (ZnAl3O4 solid solution).
The glass-ceramic substrate may be free of PbO, Na2O and K2O
In another aspect of the invention, the glass-ceramic substrate is provided by heat treating a base glass having a composition which consists in weight percent of:
and is substantially free of PbO, Na2O and K2O, said glass-ceramic substrate containing, as a main crystal phase or phases, one or more crystal phases selected from the group consisting of xcex2-quartz (xcex2-SiO2), xcex2-quartz solid solution (xcex2-SiO2 solid solution), xcex2-spodumene (xcex2-Li2O.Al2O3.4SiO2), xcex2-spodumene solid solution (xcex2-Li2O.Al2O3.4SiO2 solid solution), P-eucryptite xcex2-Li2O. Al2O3. 2SiO2, a being capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2-Li2O * Al2O3.2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO).
In another aspect of the invention, the glass-ceramic substrate is provided by heat treating a base glass having a composition which consists in weight percent of:
and is substantially free of PbO, Na2O and K2O, said glass ceramic substrate containing, as a main crystal phase, gahnite (ZnAl3O4) and/or gahnite solid solution (ZnAl3O4).
In another aspect of the invention, the glass-ceramic substrate is provided by melting glass materials, forming molten glass, annealing formed glass and heat treating the formed glass at a nucleation temperature within a range from 650xc2x0 C. to 750xc2x0 C. and a crystallization temperature within a range from 750xc2x0 C. to 950xc2x0 C., said glass-ceramic substrate having a coefficient of thermal expansion within a range from xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +20xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C.
In another aspect of the invention, the glass-ceramic substrate is provided by melting glass materials, forming molten glass, annealing formed glass and heat treating the formed glass at a nucleation temperature within a range from 650xc2x0 C. to 750xc2x0 C. and a crystallization temperature within a range from 750xc2x0 C. to 950xc2x0 C., said glass-ceramic substrate having a coefficient of thermal expansion within a range from +35xc3x9710xe2x88x927/xc2x0 C. to +80xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C.
In still another aspect of the invention, there is also provided a magnetic information recording disk provided by forming a magnetic film on a glass-ceramic substrate as described above in a state where the glass-ceramic substrate is heated at a temperature within a range from 500xc2x0 C. to 900xc2x0 C. and annealing the glass-ceramic substrate if necessary, said magnetic information recording disk having flatness of 5 xcexcm or below.
Reasons for limiting the physical properties, main crystal phases and crystal grain diameter, surface characteristics and composition of the glass-ceramics will now be described. The composition of the glass-ceramics is expressed on the basis of composition of oxides as in their base glass.
The heat resisting property will be described first. As described previously, when a magnetic medium made of an oxide such as barium ferrite which is considered suitable for a perpendicular magnetic recording medium is formed into a film on a substrate, temperature of the substrate often rises to 500xc2x0 C. or over. Besides, there is a case where the substrate formed with a film is subjected to annealing at a temperature within a range from about 500xc2x0 C. to about 900xc2x0 C. Deformation or structural change in the crystal phase or crystal grain diameter, therefore, should not take place in the substrate at such temperature. More specifically, it has been found that flatness of the substrate which is measured by a touch needle type measuring instrument after the substrate is heated 500xc2x0 C., held for 5 minutes and then cooled must be 5 xcexcm or below. The flatness of the substrate should preferably be 3 xcexcm and more preferably be 1 xcexcm. The small value of flatness should preferably be maintained even after heating at a higher temperature for a longer period of time. Preferably, the flatness of the small value should be maintained in a state after the substrate is heated to 500xc2x0 C. and held for 10 minutes, more preferably in a state after the substrate is heated to 600xc2x0 C. and held for 10 minutes and even more preferably, in a state after the substrate is heated to 700xc2x0 C. or 800xc2x0 C. and held for 10 minutes.
Description will now be made about the coefficient of thermal expansion. As described above, the film forming temperature for a magnetic recording medium made of an oxide is so high that a problem relating to a coefficient of thermal expansion in addition to the problem of deformation and structural change must be overcome. Increase in the coefficient of thermal expansion causes deformation or expansion/contraction of the medium during and after the film forming process which in turn causes change in the magnetic properties and coming off of the recording medium from the substrate. In addition, conditions under which a perpendicular magnetic recording medium is used are veried and, accordingly, the coefficient of thermal expansion of the substrate must be maintained within a substantially constant range over a temperature range from a low temperature to a high temperature. It has been found that the substrate should have a coefficient of thermal expansion within a range from xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +80xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +600xc2x0 C. The coefficient should preferably be xe2x88x925xc3x9710xe2x88x927/xc2x0 C. to +70xc3x9710xe2x88x927/xc2x0 C. and, more preferably, be xe2x88x923xc3x9710xe2x88x927/xc2x0 C. to +60xc3x9710xe2x88x927/xc2x0 C.
Description will now be made about surface roughness. As the recording density is improved as described above, the glide height of the magnetic head must be reduced. The glide height has recently been reduced to 0.025 xcexcm or below and will be further reduced to 100 xc3x85 or below and further to the near contact recording glide height. Particularly, in the ramp loading system which is suited to the high recording density, the tendency toward a lower glide height is remarkable. For these reasons, it has been found that, for realizing a lower glide height of the magnetic head, the surface roughness (Ra) of the substrate after polishing should be within a range from 1 xc3x85 to 5 xc3x85. If the surface roughness is larger than this, a desired glide height of the magnetic head in the high recording density cannot be achieved whereas if the surface roughness is smaller than this, suction between the magnetic head and the recording medium increases to such a degree as to cause damage to the recording medium.
For realizing the above described surface roughness of the substrate after polishing, it has been found that crystal grains of a crystal phase grown in the substrate should have a diameter within a range from 0.001 xcexcm to 0.10 xcexcm. If the grain diameter exceeds 0.10 xcexcm or is less than 0.001 xcexcm, a desired surface roughness cannot be attained. The grain diameter should preferably be within a range from 0.001 xcexcm to 0.07 xcexcm and, more preferably, within a range from 0.001 xcexcm to 0.05 xcexcm.
Description will now be made about crystal phases grown in the substrate. The main crystal phase is an important factor which influences the coefficient of thermal expansion. If a base glass having a coefficient of thermal expansion of a positive value is used, it is necessary to cause a crystal phase having a coefficient of thermal expansion of a negative value to grow in the substrate so that a coefficient of thermal expansion within a desired range as a whole will be realized. A main crystal phase or phases for realizing this objective is one or more crystal phases selected from the group consisting of xcex2-quartz (xcex2-SiO2), xcex2-quartz solid solution (xcex2-SiO2 solid solution), xcex2-spodumene (xcex2-Li2O.Al2O3.4SiO2), xcex2-spodumene solid solution (xcex2-Li2O.Al2O3.4SiO2 solid solution), xcex2-eucryptite (xcex2-Li2O.Al2O3.2SiO2, a part of Li2O being capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3.2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO), or gahnite (ZnAl2O4) and/or gahnite solid solution (ZnAl2O4 solid solution).
As to the main crystal phase or phases which is one or more crystal phases selected from the group consisting of xcex2-quartz (xcex2-SiO2), xcex2-quartz solid solution (xcex2-SiO2 solid solution), xcex2-spodumene (xcex2-Li2O.Al2O3.4SiO2), xcex2-spodumene solid solution (xcex2-Li2O.Al2O3.4SiO2 solid solution), xcex2-eucryptite (xcex2-Li2O.Al2O3. 2SiO2, a part of Li2O being capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2- Li2O.Al2O3.2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO), growth and ratio of one or more crystal phases selected from xcex2-quartz, xcex2-spodumene and xcex2-eucryptite are determined by the ratio of amounts of Li2O, Al2O3 and SiO2 in a specific composition range, and growth and ratio of one or more of these crystal phases xcex2-quartz, xcex2-spodumene and xcex2-eucryptite and one or more of the solid solution or solid solutions of these crystals are determined by the amounts of the other ingredients in the specific composition range. Growth and ratio of gahnite and/or gahnite solid solution are determined by the amounts of the ingredients other than ZnO and Al2O3 which constitute gahnite.
As described above, Na2O and K2O cause abnormal growth of crystal grains in the substrate and deterioration in the orientation and, accordingly, these ingredients should not be contained in the substrate. The substrate should not contain PbO either because PbO is an undesirable ingredient from the viewpoint of environment protection.
Description will now be made about respective ingredients which constitute the substrate. First, as regards the glass-ceramic substrate containing, as a main crystal phase or phases, one or more crystal phases selected from the group consisting of xcex2-quartz (xcex2-SiO2), xcex2-quartz solid solution (xcex2-SiO2 solid solution), xcex2-spodumene (xcex2-Li2O.Al2O3.4SiO2), xcex2-spodumene solid solution (xcex2-Li2O.Al2O3.4SiO2 solid solution), xcex2-eucryptite (xcex2- Li2O.Al2O3.2SiO2, a part of Li2O being capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3. 2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO) and xcex2-eucryptite solid solution (xcex2-Li2O.Al2O3.2SiO2 solid solution, a part of Li2O being capable of being substituted by MgO and/or ZnO), the SiO2 ingredient is a very important ingredient for forming these crystal phases as a main crystal phase or phases. If the amount of this ingredient is below 50%, the crystal phase grown in the glass-ceramic is instable and their texture tends to become too rough and, as a result, the mechanical strength of the substrate is reduced and the surface roughness after polishing becomes large. If the amount of this ingredient exceeds 62%, difficulty arises in melting and forming the base glass and homogeneity of the substrate is reduced. A preferable range of this ingredient is 53-57% and a more preferable range thereof is 54-56%.
The P2O5 ingredient is effective for improving the melting and refining properties of the base glass when it coexists with the SiO2 ingredient. If the amount of this ingredient is below 5%, these effects cannot be obtained whereas if the amount of this ingredient exceeds 10%, resistance to devitrification is reduced and, as a result, the texture of the glass-ceramic in the crystallization stage becomes too rough and the mechanical strength of the substrate is reduced. A preferable range of this ingredient is 6-10% and a more preferable range thereof is 7-9%.
For achieving the above effects more significantly, it is preferable to include SiO2+P2O5 in the amount of 61-65% with the ratio of P2O5/SiO2 within a range from 0.12 to 0.16.and it is more preferable to include SiO2+P2O5 in the amount of 62-64% with the ratio of P2O5/SiO2 within a range from 0.13 to 0.15.
If the amount of the Al2O3 ingredient is below 22%, difficulty arises in melting of the base glass and, accordingly, homogeneity of the glass-ceramic obtained is reduced and chemical durability of the glass-ceramic is deteriorated. If the amount of this ingredient exceeds 26%, difficulty also arises in melting of the base glass and, as a result, homogeneity is reduced and resistance to devitrification of the base glass is reduced and, as result, the texture of the glass-ceramic becomes too rough in the crystallization stage and the mechanical strength of the substrate is reduced. A preferable range of this ingredient is 23-26% and a more preferable range thereof is 23-25%.
The Li2O, MgO and ZnO ingredients are important ingredients for forming xcex2-quartz, xcex2-spodumene, xcex2-spodumene solid solution, xcex2-eucryptite, and xcex2-eucryptite solid solution. These three ingredients are important also in that, by coexistence of the above described SiO2 and P2O5 ingredients, these three ingredients improve the low expansion characteristic, reduce the amount of flexion at a high temperature and significantly improve melting and refining properties of the base glass.
If the amount of the Li2O ingredient is below 3%, the above described effect cannot be obtained and, moreover, homogeneity is reduced as a result of reduction of the melting property and growth of a desired crystal phase becomes difficult. If the amount of this ingredient exceeds 5%, the low expansion characteristic cannot be obtained and resistance to devitrification of the base glass is reduced which causes the texture of the glass-ceramic to become too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this ingredient is 3.5-5% and a more preferable range thereof is 3.5-4.5%.
If the amount of the MgO ingredient is below 0.5%, the above described effect cannot be obtained whereas if the amount of this ingredient exceeds 2%, the low expansion characteristic cannot be obtained. A preferable range of this ingredient is 0.5-1.8% and a more preferable range thereof is 0.6-1.5%.
If the amount of the ZnO ingredient is below 0.2%, the above described effect cannot be obtained whereas if the amount of this ingredient exceeds 2%, the low expansion characteristic cannot be obtained and, moreover, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage and the mechanical strength thereby is reduced. A preferable range of this ingredient is 0.2-1.8% and a more preferable range thereof is 0.2-1.5%.
For achieving the above described effects more significantly, the total amount of the three ingredients of Li2O, MgO and ZnO should be within a range from 4.0% to 6.5% and, preferably within a range from 4.3% to 6.5% and, more preferably, within a range from 4.5% to 6.5%.
The two ingredients of CaO and BaO are ingredients which remain as a glass matrix other than the crystal in the glass-ceramic. These ingredients are important for fine adjustment of the cyrstal phase and the glass matrix phase for improving the above described low expansion characteristic and melting property. If the amount of CaO is below 0.3%, this effect cannot be obtained whereas if the amount of this ingredient exceeds 4$, a desired crystal phase cannot be obtained and, moreover, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this ingredient is 0.5-3% and a more preferable range thereof is 0.5-2%.
If the amount of the BaO ingredient is below 0.5%, the above effect cannot be obtained whereas if the amount of this ingredient exceeds 4%, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this ingredient is 0.5-3% and a more preferable range thereof is 0.5-2%.
For improving the above described effects more significantly, the total amount of CaO+BaO should be within a range from 0.8% to 5% and, preferably within a range from 1% to 4% and, more preferably, within a range from 1% to 3%.
The TiO2 and ZrO2 ingredients are indispensable as nucleating agents. If the amount of each ingredient is below 1%, a desired crystal cannot be grown whereas if the amount of each ingredient exceeds 4%, the melting property of the base glass is reduced and homogeneity thereby is reduced and, in the worst case, an unmolten ingredient is left in the glass-ceramic. A preferable range is 1.5-4% for TiO2 and 1.5-3.5% for ZrO2 and a more preferable range is 1.5-3.5% for TiO2 and 1-3% for ZrO2.
The As2O3 and Sb2O3 ingredients may be added as a refining agent in melting the materials of the glass for obtaining a homogeneous product. It will suffice if one or both of these ingredients are added up to the total amount of 4%. A preferable amount of As2O3+Sb2O3 is 0-2% and, more preferably As2O3 in the amount of 0-2%.,
For achieving fine adjustment of the properties of the glass-ceramic or other purpose, one or more of SrO, B2O3, F2, La2O3, Bi2O3, WO3, Y2O3, Gd2O3 and SnO2 ingredients may be added in the total amount of 2% or below in addition to the above described ingredients. One or more of coloring agents such as CoO, NiO, MnO2, Fe2O3 and Cr2O3 may also be added in the total amount of 2% or below.
As to the glass-ceramic substrate containing, as a main crystal phase, gahnite and/or gahnite solid solution provided in another aspect of the invention, if the SiO2 ingredient is below 30%, crystal grains tend to become coarse and chemical durability and mechanical strength are reduced whereas if the amount of this ingredient exceeds 65%, difficulty arises in melting of the base glass and homogeneity thereby is reduced. A preferable range of this ingredient is 32-63% and a more preferable range thereof is 34-61%.
If the amount of the Al2O3 ingredient is below 5%, difficulty arises in the growth of gahnite as a main crystal phase whereas if the amount of this ingredient exceeds 35%, decrease in homogeneity due to deterioration in the melting property occurs and resistance to devitrification is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this amount is 7-33% and a more preferable range thereof is 10-30%.
The ZnO ingredient is an important ingredient for forming, together with the Al2O3 ingredient, gahnite as a main crystal phase and for improving the mechanical strength and heat resistance property. If the amount of this ingredient is below 5%, the above effects cannot be obtained whereas if the amount of this ingredient exceeds 35%, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this ingredient is 7-33% and a more preferable range thereof is 10-30%.
If the amount of the MgO ingredient is below 1%, homogeneity is reduced due to deterioration in the melting property of the base glass and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. If the amount of this ingredient exceeds 20%, resistance to devitrification of the base glass is reduced. A preferable range of this ingredient is 3-18% and a more preferable range thereof is 3-15%.
The CaO, SrO, BaO, B2O3, La2O3, Y2O3, Gd2O3, Ta2O5, Nb2O5, WO3 and Bi2O3 ingredients are effective for improving the melting property of the base glass. Particularly, the La2O3, Y2O3, Gd2O3, Ta2O5, Nb2O5, WO3 and Bi2O3 ingredients are also effective for improving the mechanical strength and chemical durability of the product. For achieving these effects while preventing the grown crystal phase from becoming too rough in the heating process, the total amount of one or more of these ingredients should be within a range from 0.5% to 20%. If, however, the amount of the B2O3 ingredient exceeds 10% or if the total amount of one or more of the Ta2O5, Nb2O5, WO3 and Bi2O3 ingredients exceeds 10%, difficulty arises in the growth of a desired crystal phase. The total amount of CaO+SrO+BaO+B2O3+La2O3+Y2O3 +Gd2O3+Ta2O5+Nb2O5+WO3+Bi2O3 should preferably be within a range from 0.5% to 15%, the amount of B2O3 should preferably be within a range from 0% to 8% and the total amount of Ta2O5,+Nb2O5+WO3+Bi2O3 should preferably be within a range from 0% to 5%. A more preferable range of the total amount of CaO+SrO+BaO+B2O3+La2O3+Y2O3+Gd2O3+Ta2O5+Nb2O5+WO3+Bi2O3 is 0.5-10%, a more preferable range of the Bi2O3 ingredient is 0-5% and a more preferable range of the total amount of Ta2O5 +Nb2O5+WO3+Bi2O3 is 0-5%.
The TiO2 ingredient is indispensable as a nucleating agent. If the amount of this ingredient is below 1%, a desired crystal phase cannot be grown whereas if the amount of this ingredient exceeds 15%, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable range of this ingredient is 3-13% and a more preferable range thereof is 4-10%.
The ZrO2, P2O5 and SnO2 ingredients may be added as an auxiliary nucleating agent. If the total amount of one or more of these ingredients exceeds 7%, and if the amount of the respective ingredients exceeds 2%, 5% and 2%, resistance to devitrification of the base glass is reduced and, as a result, the texture of the glass-ceramic becomes too rough in the crystallization stage with resulting decrease in the mechanical strength. A preferable total amount of ZrO2+P2O5+SnO2 is 6% or below, the amount of ZrO2 is below 1.8%, the amount of P2O5 is 4.5% or below and the amount of SnO2 is 1.8% or below. A more preferable total amount of ZrO2+P2O5+SnO2 is 5% or below, the amount of ZrO2 is below 1.7%, the amount of P2O5 is 4% or below and the amount of SnO2 is 1.7% or below.
The As2O3 and/or Sb2O3 ingredients may be added as a refining agent in melting of the base glass. It will suffice if one or both of these ingredients are added up to the total amount of 4%. A preferable total amount is 3% or below and a more preferable total amount is 2% or below.
If one or more fluorides of the above ingredients are contained in the substrate, it will be effective for melting of the base glass and adjusting crystallization. If the total amount of F of these fluorides exceeds 5%, the tendency to devitrification increases and a good product cannot be obtained.
In addition to the above described ingredients, coloring agents such as MnO2, NiO, CoO, Fe2O3, Cr2O3, V2O5, MoO2 and Cu2O, and GeO2 and other rare earth oxide than the above described ones may be added in the total amount of one or more of these ingredients up to 10% within a range which will not impair the desired properties of the product.
In each of the above described glass-ceramic substrates of the two systems according to the invention, the total amount of the above described ingredients should be 90% or over, preferably 95% or, more preferably 98%, for providing an excellent glass-ceramic substrate.
For manufacturing each of the glass-ceramic substrate for a magnetic information recording medium of the two systems according to the invention, the base glass having the above described composition is melted, is subjected to heat forming and/or cold forming, is heat treated for producing a crystal nucleus under a temperature within a range from 650xc2x0 C. to 750xc2x0 C. for about one to twelve hours, and further is heat treated for crystallization under a temperature within a range from 750xc2x0 C. to 950xc2x0 C. for about one to twelve hours.